Median cross-calibration bias parameter, J₁ (Eq. (3)), of different observations and their statistical uncertainties obtained using Method 1 shown as red crosses for the MOS1/pn (left panel) and MOS2/pn (right panel) pairs (at the lowest energies, the values are smaller than the plot symbol width). The blue crosses indicate the standard deviation of the cluster sample at a given energy. The black crosses indicate the medians and the range containing 68% of the values of the 3XMM sample. The horizontal dashed line indicates the expectation (unity) in the case of no cross-calibration bias.